Abstract
Background: Crowding refers to the failure to identify a peripheral object because other objects (flankers) surround it. A hallmark characteristic of crowding is the inner-outer asymmetry, i.e., the unintuitive fact that the outer flanker produces more substantial interference than the inner one. Despite recent efforts to explain this characteristic of crowding, the processes that underlie the inner-outer asymmetry are still unclear. Here, we investigated the role of attention in visual crowding by investigating whether and how spatial transient attention interacts with its flankers' asymmetrical effect. Method. Eighteen observers estimated the orientation of a Gabor target presented at 7° eccentricity. The crowding display consisted of two flankers along the horizontal meridian, one on each side of the target. We manipulated attention by using a pre-cue that could appear at one of four possible locations: fixation, target, inner-flanker, or outer flanker. We assessed each flanker's contribution to the pattern of errors by fitting probabilistic mixture-models. Results. Consistent with our previous findings, observers often misreported the outer (eccentric) flanker as the target (reflecting the inner-outer asymmetry). Interestingly, directing transient attention to the inner flanker location reduced crowding interference by decreasing the outer flanker reports. However, directing attention to the outer flanker location increased the crowding interference by increasing the outer flanker reports. Conclusions. The present results are inconsistent with some of the current crowding models (e.g., the cortical magnification and the receptive size views). Our findings suggest that spatial attention plays an essential role in the inner-outer asymmetry, a hallmark characteristic of crowding.